1. Field of the Invention
The invention relates to ceramic igniter compositions, and more particularly, to such compositions that contain components of a conductive material and insulating material, where the insulating material component includes a relatively high concentration of metal oxide.
2. Background
Ceramic materials have enjoyed great success as igniters in gas fired furnaces, stoves and clothes dryers. Ceramic igniter production requires constructing an electrical circuit through a ceramic component, a portion of which is highly resistive and rises in temperature when electrified by a wire lead.
One conventional igniter, the Mini-Igniter™, available from the Norton Igniter Products of Milford, N.H., is designed for 12 volt through 120 volt applications and has a composition comprising aluminum nitride (“AIN”), molybdenum disilicide (“MoSi2”), and silicon carbide (“SiC”). However, while the Mini-Igniter™ is a highly effective product, certain applications require voltages in excess of 120 V.
In particular, in Europe, nominal voltages include 220 V (e.g. Italy), 230 V (e.g. France), and 240 V (e.g. U.K.). Standard igniter approval tests require operation at a range of from 85 percent to 110 percent of a specified nominal voltage. Thus, for a single igniter to be approved for use throughout Europe, the igniter must be operational from about 187 to 264 V (i.e. 85% of 220 V and 110% of 240 V). Current igniters have difficulty providing such a high and extended voltage range, particularly where a relatively short hot zone length (e.g. about 1.2 inches or less) is employed.
For instance, at higher voltage applications, current igniters may be subject to temperature runaway and thus require a transformer in the control system to step down the voltage. Use of such a transformer device is clearly less desirable. Accordingly, there is a need for relatively small igniters for high voltage applications, particularly over a range of from about 187 to 264 V, which do not require an expensive transformer but still possess the following requirements set by appliance and heating industries to anticipate variation in line voltage:
Time to temperature (“TTT”)<5 secMinimum temperature at 85% of design voltage1100° C.Design temperature at 100% of design voltage1300° C.Maximum temperature at 110% of design voltage1500° C.Hot-zone Length<1.2–1.5″Power<100 W.
For a given igniter geometry, one possible route to provide a higher voltage system is by increasing the igniter's resistance. The resistance of any body is generally governed by the equation
Rs = Ry × L/A,whereinRs = Resistance;Ry = Resistivity;L = the length of the conductor; andA = the cross-sectional area of the conductor.
Because the single leg length of current ceramic igniters is about 1.2 inches, the leg length can not be increased significantly without reducing its commercial attractiveness. Similarly, the cross-sectional area of the smaller igniter, between about 0.0010 and 0.0025 square inches, will probably not be decreased for manufacturing reasons.
U.S. Pat. No. 5,405,237 (“the Washburn patent”) discloses compositions suitable for the hot zone of a ceramic igniter comprising (a) between 5 and 50 volume % (“v/o” or “vol %”) MoSi2, and (b) between 50 and 95 v/o of a material selected from the group consisting of silicon carbide, silicon nitride, aluminum nitride, boron nitride, aluminum oxide, magnesium aluminate, silicon aluminum oxynitride, and mixtures thereof.
Additional highly useful ceramic compositions and systems are disclosed in U.S. Pat. Nos. 5,514,630 and 5,820,789, both to Willkens et al. U.S. Pat. No. 5,514,630 reports that hot zone compositions should not exceed 20 v/o of alumina. U.S. Pat. No. 5,756,215 reports additional sintered compositions that include lead layers that contain up to 2% by weight of silicon carbide.
It thus would be desirable to have new ceramic hot zone igniter compositions. It would be particularly desirable to have new igniter compositions that could reliably operate at high voltages, such as from about 187 to 264 V, especially with a relatively short hot zone length.